Note: Descriptions are shown in the official language in which they were submitted.
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SECURITY DEVICE
The invention relates to a security device and to a security document
provided with such a security device.
A variety of security devices have been proposed in the past to prevent
security documents from being counterfeited or fraudulently produced. A
particularly useful security device is one which is readily verifiable by a
user but
which is difficult to produce. One example of such a security device is a
clear
transparent region in an otherwise opaque substrate. The use of a clear
transparent region prevents the generation of a "simple" counterfeit arising
from
the increasing popularity of colour photocopiers and other imaging systems and
the improving technical quality of colour photocopies. In addition the clear
transparent region provides a feature that is easily verifiable by the general
public. However a clear transparent region in an opaque substrate is
susceptible to counterfeiting, for example by punching a hole in an opaque
substrate and then placing a clear transparent polymeric film over the hole.
In the prior art this problem has been addressed by the use of additional
optically variable security devices in the clear transparent regions. There
are
numerous examples in the prior art of applying a reflection-based diffractive
device in the window of a banknote. For example US-A-6428051 discloses the
use of a diffractive device combined with a reflective metallised layer.
However
in such devices the image is visible in reflected light and distracts the eye
from
verifying the presence of a clear transparent region.
WO-A-99/37488 describes the use of a diffractive optical element in a
clear transparent region, such that when collimated light passes through the
diffractive optical element it is transformed by the diffractive structure
into a
recognisable pattern by the process of diffraction. The requirement for a
collimated light source means that this feature is not easily verifiable by
the
general public and it is more appropriate for verification by bank tellers and
retail
staff with appropriate equipment and training.
Another example of a known security device is described in WO-A-
01/02192. In this case, first and second diffractive structures or gratings
are
formed in respective first and second zones of a transparent window. The
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diffractive structures are chosen to diffract particular wavelengths of light
outside
of the users field of view leaving selected wavelengths within the users field
of
view, the wavelengths within the field of view producing visually discernible
colours which together form a projected security image. In this device the
projected security image, defined by the diffracted light, is visible at most
common angles of view when the device is viewed in transmission.
In accordance with the present invention, we provide a security device
comprising a substrate having a transparent region, wherein at least one
optical
element is provided in part of the transparent region, the optical element
causing
an incident off-axis light beam transmitted through the optical element to be
redirected away from a line parallel with the incident light beam whereby when
the device is viewed in transmission directly against a backlight, the
presence of
the optical element cannot be discerned but when the device is moved relative
to
the backlight such that lines of sight from the viewer to the transparent
region
and from the transparent region to the backlight form an obtuse angle at which
redirected light is visible to the viewer, a contrast is viewed between the
part of
the transparent region including the optical element and another part of the
transparent region, and wherein when the security device is viewed in
reflection
under diffuse lighting conditions either no contrast can be discerned between
the
two parts or a different contrast can be discerned between the two parts.
The invention provides an improved security device in a clear transparent
region that is simple to verify when viewed in transmitted light. The security
device of the current invention uses one or more optical elements to create an
apparent silhouette of an opaque image in an optically transmissive region,
typically incorporated into a secure document. The apparent silhouette of the
image appears in the plane of the transparent region when viewed under
particular conditions. The security device is optically variable in the sense
that
when it is viewed in diffuse light, or directly backlit by a source that is
aligned
with the device and the observer, the image is essentially invisible, and the
window appears transparent and featureless. However, when the backlit
transparent region is viewed such that it forms the appropriate range of
obtuse
angles between the viewer and the light source the apparent silhouette of the
image appears. A further important aspect of this security device is that the
image cannot be detected when the device is viewed under reflected light. The
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fact that the image is not viewed in reflection under diffuse lighting
conditions
further increases the security of the device by making it impossible to mimic
the
silhouette of the image using conventional printing techniques which by their
nature are visible in reflection and transmission.
In contrast to the device of WO-A-01/02192 there is an intentional
optically variable effect and there is interaction between the user and the
device
to reveal the security image. One advantage of the security device according
to
the invention is that the method of authentication, which uses a simple
interaction between the user and the device, makes the device easily
recognisable and memorable to the user and therefore increases its counterfeit
resistance.
The optical element(s) can take a variety of forms. In the most preferred
examples, the optical element is substantially transparent and may comprise a
diffraction grating. This is convenient because diffraction gratings have a
first
order component at a sufficiently large angle to the zero order to maximise
the
contrast effect. Preferably a diffraction grating is chosen such that the
middle of
the range of obtuse angles a between the viewer and the light source for the
redirected diffracted beam is less than 1800 but greater than 90 and more
preferably in the range 130-175 and even more preferably in the range 150-
170 . The degree of diffraction will depend on the wavelength of the incident
beam and therefore for a polychromatic light source the redirected light will
be
spread over an angular range where the redirected red light defines the upper
end of the range of obtuse angles between the viewer and the light source and
the redirected blue light defines the lower end. Preferably a diffraction
grating is
chosen such that the angular spread of the diffracted light is up to 60 and
more
preferably between 1-25 and even more preferably between 5-15 . In order to
achieve the diffractive conditions defined above a linear grating can be
employed with a line density in the range 200-1500 lines/mm and more
preferably in the range 250-1000 lines/mm and even more preferably in the
range 300-700 lines/mm.
In another example, the or each optical element is formed by a set of
spaced prismatic elements.
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In this case, each of a first set of elements will typically have opposed
sets of facets, one set of the facets being reflective to visible light and
the
opposed set of facets being absorbent to visible light. Typically, the device
will
further include a set of spaced prismatic elements with opposed opaque facets.
The contrast between the two parts which is observed can be designed in
a variety of ways. For example, a simple geometric or graphical shape could be
used but in the preferred examples, a recognisable image is defined such as
pictorial images, patterns, symbols and alphanumeric characters and
combinations thereof. Possible characters include those from non-Roman
scripts of which examples include but are not limited to, Chinese, Japanese,
Sanskrit and Arabic. It should be understood that the shape of the image may
be defined by the optical element itself when one such element is provided or
by
the "another part" of the transparent region, typically defined between two or
more optical elements.
In certain preferred examples, the security device further comprises a
printed or metallised permanent image on the transparent region. The
permanent image may take any form but typical examples include patterns,
symbols and alphanumeric characters and combinations thereof. The permanent
image can be defined by patterns comprising solid or discontinuous regions
which may include for example line patterns, fine filigree line patterns, dot
structures and geometric patterns. Possible characters include those from non-
Roman scripts of which examples include but are not limited to, Chinese,
Japanese, Sanskrit and Arabic. The radiation used for viewing the indicia
would
typically be in the visible light range but could include radiation outside
the
visible range such as infrared or ultraviolet. For additional security, this
permanent image may cooperate with a recognisable image formed by the said
contrast.
In an alternative embodiment the security device further comprises a
reflective based optically variable device such as a hologram or diffraction
grating. These devices are commonly formed as relief structures in a
substrate,
which is then provided with a reflective coating to enhance the replay of the
device. The reflective based optically variable device is part of the
transparent
region and in order to maintain the transparency of the security device the
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reflective coating is provided by a reflection enhancing material which is
substantially transparent. Suitable transparent reflection enhancing materials
include high refractive index layers for example ZnS. Further suitable
transparent reflection enhancing materials are referred to in EP201323.
The reflective based optically variable device is optimized for operation in
reflection. This is in contrast to the diffraction grating use to form the
optical
element which is optimized for operation in transmission. An important
distinction between reflection and transmission diffractive microstructures
(diffraction gratings, holograms, etc) is the depth at which optimum
diffraction
efficiency is achieved. For a reflection structure the optimum embossing depth
is
approximately equal to the optical wavelength divided by 3n, where n if
the refractive index. Whereas, for a transmission structure there is a(n/(n-
1))
multiplier which results in a peak efficiency at embossing depths that are
typically three times deeper than that for a reflective structure. Thus when a
diffractive structure is optimised for high reflection efficiency it's
diffractive
efficiency in transmission is necessarily poor.
Typically, the or each optical element is embossed into the substrate or
into an embossing lacquer applied to the substrate although the invention is
equally applicable to optical elements which have been adhered to a
transparent
substrate such as via a transfer process or the like.
In most cases, the backlight will be formed by a light source located
behind the device. However, the backlight could be formed by a reflector, such
as a white surface.
Security devices according to the invention can be used to secure a wide
variety of articles but are particularly suitable for inclusion in a security
document. In that case, the security device could be adhered to the document
but preferably the substrate of the security document provides the substrate
of
the security device.
In the case of security documents, the recognisable image produced by
the contrast may relate to an image found elsewhere on the security document.
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Some examples of security devices and security documents according to
the invention will now be described with reference to the accompanying
drawings, in which:-
Figures 1A and 1B illustrate schematically a first example of a security
device according to the invention when viewed in two different ways and
illustrating the appearance of the device in each case;
Figures 2A and 2B are similar to Figures 1A and 1 B respectively but of a
second example;
Figures 3A and 3B illustrate a security document incorporating a first
example of the security device when viewed under different conditions;
Figures 4 to 7 illustrate four further examples of security documents;
Figures 8-10 illustrate examples of security devices also comprising a
reflective diffractive device; and,
Figure 11 illustrates a security device also comprising a reflective
diffractive device and a permanent metallised image.
A first example of a security device according to the invention is shown in
Figures 1A and 1 B. This device comprises a transparent region 1 of a
substrate
into respective, spaced parts of which have been embossed optical elements
2,3. An unembossed part 4 is located between the optical elements 2,3. In this
case, the unembossed part 4 defines an image under certain viewing conditions.
When the device is directly backlit, such that a light source 6, which is of
higher intensity than the ambient light level is in-line with the device and
the
observer, the intensity of the transmitted light through both the optical
elements
2,3 and the non-deflecting region(s) 4 appears substantially the same to the
viewer such that the transparent region appears substantially transparent and
featureless (see resultant image in Figure la).
When the device is panned away from the light source 6 (Figure 113),
such that the observer is no longer viewing the device in the direction of the
light
source 6, a range of viewing angles (a) are achieved at which the optical
elements 2,3 redirect light from the source 6 back towards the observer
resulting
in the areas that contain the optical elements appearing brightly illuminated.
In
contrast, in the non-deflecting regions 4, the light is not redirected, and
the
observer simply sees ambient light transmitted through the clear transparent
region 4. For a wide range of viewing angles and backlight conditions, the
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contrast between the redirected light and the ambient light gives the
impression
that there is a real obstruction in the transparent region 4. In this example
the
silhouette is in the shape of a traditional elongate banknote security thread.
The
obstruction is observed in the transparent region as a silhouette in the form
of
the image defined by the non-deflecting region(s) 4 (see resultant image in
Figure 1 b). The observer authenticates the feature by holding the note up to
a
backlight and panning from side to side away from the light source. This then
alternately generates and hides the apparent image.
The optical elements 2,3 should be capable of efficiently bending or
redirecting light to viewing angles off-axis (i.e. the incident light does not
impinge
on the device in a direction perpendicular to the plane of the device), whilst
allowing (at least partial) direct transmission when the source, observer and
device are directly aligned. In a preferred (but not sole) embodiment the
optical
elements are linear diffraction gratings. If the gratings 2,3 are formed in or
transferred to the transparent substrate 1 then they will appear essentially
transparent when held directly to the light, however when moved from side to
side, such that the observer is positioned in the first order diffraction
region, light
from the source 6 will be diffracted towards the viewer at an angle dictated
by
the wavelength. This wavelength dependence thus gives a further enhancement
to the feature described in Figure 1 whereby the silhouette of the image is
consequently seen to be backlit by a changing array of colours when the
viewing
position is varied. It can be seen that as the device is moved a range of
obtuse
angles a i s subtended between the viewer and the source 6 at the non-
deflecting region 4. As explained above, a varies between 90 and 130 ,
preferably 130-175 , most preferably 150-170 . When viewed in reflection under
diffuse conditions the reflected light from the diffractive and non-
diffractive
regions is of a similar intensity because firstly the diffraction gratings are
optimised for transmitted light and therefore the efficiency of the reflective
diffractive component is low and secondly any residual non-zero (reflected)
orders are continuously distributed and superimposed.
A second example of a security device according to the invention is
shown in Figures 2A and 2B. The device comprises a transparent region of a
substrate into respective, spaced parts of which have been replicated
deflecting
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optical elements 10,11 comprising an array of linear prisms 10A,11A
respectively, the individual prisms being spaced apart so as to define planar
parts 13 between them.
Each prism IOA and 11A has a pair of opposed facets 10B,10C; 11B,
11 C. Corresponding facets 10B,11 B; 10C,11 C are parallel.
The facets 10B and 11 B are provided with a black, fully light absorbent
coating. The facets 10C and 11 C are formed with a reflective coating such as
a
preferential metallization of for example aluminium.
A non-deflecting 11 prismatic structure 12, comprising an array of prisms
12A, is located between the optical elements 10 and 11 and defines an image
under certain viewing conditions. As with optical elements 10 and 11 the
individual prisms are spaced apart so as to define planar parts 13 between
them.
Each prism 12A has a pair of opposed facets 12B and 12C. The facets 12B and
12C are provided with a black, fully light absorbent coating.
When viewed in reflection, the device will present a substantially uniform
appearance as the light incident on the prisms 10A, 11A and 12A will either be
absorbed by the black coating on the facets 12B or 12C or be reflected by the
reflective facets 10C and 11 C onto the opposed black coating on facets 10B
and
11 B respectively. Light incident on the regions 13 will simply pass through
to the
underlying background. The width (x) of the linear prisms 10A, 11A and 12A
and the planar regions 13 are such that they cannot be resolved with the naked
eye and therefore provides a uniform appearance in reflection. Typical
dimensions for the width of the linear prisms and the width of the planar
regions
are in the range 25-200 microns and more preferably in the range 50-100
microns.
When the device is directly backlit and viewed in transmission such that
the observer, security device and backlight 14 are aligned (Figure 2a), both
the
deflecting optical elements 10,11 and the non-deflecting optical element 12
allow
partial transmission of the light through the planar transparent regions 13.
The
individual prisms 10A, 11A and 12A absorb light for the same reasons as
described for the device in reflective mode. The small non-resolvable size of
the
individual prisms 10A, 11A and 12A and the planar regions 13 result in the
device appearing uniformly translucent (see resultant image in Figure 2a).
When the device is viewed away from the light source such that the observer is
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no longer viewing the device in the direction of the light source 14 an
appropriate
viewing angle a is reached where light is redirected by the reflective facets
10C
and 11 C(Figure 2b). In contrast in the non-deflecting prismatic structure 12,
where the reflective surfaces are absent, the light is not redirected, and the
observer simply sees ambient light partially transmitted through the prismatic
structure 12. The contrast between the deflecting and non-deflecting regions
results in a silhouette of the image appearing in the non-deflecting regions
12
(see resultant image in Figure 2b). In this example the silhouette is in the
shape
of a traditional elongate banknote security thread.
Examples of security documents with which the present invention can be
used include banknotes, fiscal stamps, cheques, postal stamps, certificates of
authenticity, articles used for brand protection, bonds, payment vouchers, and
the like.
The security document (or security device) may have a substrate formed
from any conventional material including paper and polymer. Techniques are
known in the art for forming transparent regions in each of these types of
substrate. For example, WO-A-8300659 describes a polymer banknote formed
from a transparent substrate comprising an opacifying coating on both sides of
the substrate. The opacifying coating is omitted in localised regions on both
sides of the substrate to form a transparent region.
WO-A-0039391 describes a method of making a transparent region in a
paper substrate in which one side of a transparent elongate impermeable strip
is
wholly exposed at one surface of a paper substrate in which it is partially
embedded, and partially exposed in apertures at the other surface of the
substrate. The apertures formed in the paper can be used as the first
transparent region in the current invention.
Other methods for forming transparent regions in paper substrates are
described in EP-A-723501, EP-A-724519 and WO-A-03054297.
There is no limitation on the image defined by the non-deflecting regions,
and the examples discussed below are not intended to limit the invention.
Figure 3 illustrates one example of a security document such as a
banknote 20. A transparent region 21 is formed in an opaque substrate 22. Two
optical elements 23,24, in the form of diffraction gratings, are present in
the left
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and right portions of the transparent region 21, separated by a non-deflecting
optically transparent region 25. Each diffraction grating 23,24 is such that
it
exhibits straight through (zeroth order) transmission and generates spectrally
well spread first order diffraction regions that occur at a sufficient angular
displacement to generate a high level of contrast between the ambient light
level
and the diffracted rays. The non-deflecting region 25 defines the image and is
in
the shape of a traditional elongate banknote security thread. Viewed in
transmission when the light source, transparent region 21 and the observer are
in alignment, the transparent region 21 appears uniformly transparent and the
image is hidden (Figure 3A). When the substrate 22 is panned away from the
light source the regions of the transparent region that contain the
diffractive
optical elements 23,24 appear brightly illuminated but in contrast the non-
deflecting region 25, transmitting ambient light, appears dark and the
silhouette
of the thread is revealed (Figure 3B).
The optical elements and non-deflecting regions can be arranged such
that the image appears as a traditional elongate banknote windowed thread, as
illustrated in Figure 4. Alternatively a series of alphanumeric images could
be
defined along the transparent region, again if desired to give the impression
of a
security thread, as illustrated in Figure 5.
In a further example shown in Figure 6 the transparent region comprises
a printed image, in the form of an array of stars, that combines with a
silhouette
image, in the form of a wavy line, to form a further complete image. On
holding
the substrate up to a backlight and panning from side to side the observer
will
observe a permanent printed image and the appearance and disappearance of a
second image formed by the combination of the permanent printed image and
the silhouette. The permanent image could be printed using lithography, UV
cured lithography, intaglio, letterpress, flexographic printing, gravure
printing or
screen printing. Alternatively the permanent image can be created using known
metallisation or demetallisation processes.
In a further example the silhouette image is linked to the image printed on
the secure substrate. Figure 7 illustrates an example where the image printed
on the note is completed by the silhouette image, thereby providing a clear
link
between the transparent region and the secure document it is protecting.
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Figures 8A, 8B and 8C illustrate a further example in which the security
device also comprises a reflective diffractive device, which in this example
is in
the form of a hologram which replays in reflected light as an array of stars.
The
device, illustrated in cross-section in Figure 8a, comprises a transparent
region
30 of a substrate 31 on to which has been applied an embossing lacquer 32 into
respective, spaced parts of which have been embossed two optical elements
33,34, in the form of diffraction gratings, separated by an unembossed non-
deflecting optically transparent region 35. The diffraction grating for the
optical
elements 33,34 is such that it exhibits straight through (zeroth order)
transmission and generates spectrally well spread first order diffraction
regions
that occur at a sufficient angular displacement to generate a high level of
contrast between the ambient light level and the diffracted rays. A
holographic
structure 36 optimised for operation in reflected light is embossed into the
embossing lacquer along both edges of the transparent region. A high
refractive
index layer 37, for example vapour deposited ZnS, is applied over the
embossing lacquer such that it covers the whole of the transparent region.
Alternatively the high refractive index layer could be applied solely over the
holographic embossing.
The reflective diffractive device is optimised for reflective light and
therefore its diffraction efficiency in transmission is poor such that in
transmitted
light it acts as a further non-deflecting region. When the light source,
transparent region and the observer are in alignment the holographically
embossed region, the diffractive optical elements 33,34 and the unembossed
region 35 appear uniformly transparent. (Figure 8B). When the substrate is
panned away from the light source the regions of the transparent region that
contain the diffractive optical elements 33,34 appear brightly illuminated but
in
contrast the unembossed region 35 and the holographically embossed regions
36, both acting as non-deflecting regions and transmitting ambient light,
appear
dark revealing the silhouette of a central thread and the silhouette defining
an
outline of the holographic image array (Figure 8C). When the substrate is
viewed in reflection the silhouette image generated by the non-deflecting
region
35 disappears but the holographic image becomes readily apparent, due to the
presence of the high refractive index reflection enhancing layer 37, and the
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hologram 36 replays as an array of stars along both edges of the transparent
region (Figure 8D).
The security device illustrated in Figure 8 couples the advantage of
maintaining a completely transparent region when directly backlit with the
additional security of displaying a different optically variable image when
viewed
in transmitted and reflected light.
Figures 9A-9D illustrate a further example of a security device similar to
Figure 8 but in which the sole non-deflecting region 40 is formed from a
combination of unembossed and holographically embossed areas 41,42. The
device, illustrated in cross-section in Figure 9A, comprises a transparent
region
30 of a substrate 31 on to which has been applied an embossing lacquer 32 into
respective, spaced parts of which have been embossed two optical elements
33,34, in the form of diffraction gratings, separated by the non-deflecting
region
40 which is substantially non-deflecting to transmitted light. The diffraction
grating for the optical elements is as described for Figure 8. The non-
deflecting
region 40 defines the image and is in the shape of a traditional elongate
banknote security thread. As with the example in Figure 8 the holographic
structure 42 is optimised for operation in reflected light.
When the light source, transparent region and the observer are in
alignment the non-deflecting region 40 and the diffractive optical elements
33,34
appear uniformly transparent (Figure 9B). When the substrate is panned away
from the light source the transparent regions that contain the diffractive
optical
elements 33,34 appear brightly illuminated but in contrast the unembossed
region 40 and the holographically embossed region 41, both acting as non-
deflecting regions and transmitting ambient light, appear dark and the
silhouette
of a central thread is revealed (Figure 9C). The holographic image is not
apparent in transmitted light due to the negligible contrast between the
unembossed and holographically embossed regions but in reflection the
silhouette image of the thread disappears to reveal a hologram replaying as a
line of stars down the centre of the transparent region (Figure 9D).
Figures 10A-10D illustrate a further example of the security device of the
current invention in which an additional reflective diffractive device in the
form of
a hologram is incorporated. The device, illustrated in cross-section in Figure
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10A, comprises a transparent region 30 of a substrate 31 on to one side of
which
has been applied an embossing lacquer 32 into respective, spaced parts of
which have been embossed two optical elements 33,34, in the form of
diffraction
gratings, separated by a unembossed non-deflecting optically transparent
region
40. The diffraction grating for the optical elements is as described for
Figure 8.
The non-deflecting region 40 defines the image and is in the shape of a
traditional elongate banknote security thread. A second layer 50 of embossing
lacquer is applied to the opposite side of the transparent substrate 31 and a
holographic structure 51, optimised for operation in reflected light, is
embossed
into the embossing lacquer such that it covers the majority of the transparent
region. A high refractive index layer 37, for example vapour deposited ZnS, is
applied over the second layer of embossing lacquer such that it covers the
whole
of the transparent region.
When viewed in transmitted light, with the viewer on either side of the
device, the device will operate in the same manner as described in reference
to
Figure 1. This is because the holographic structure optimised for operation in
reflected light has negligible effect on the transmitted light. When the light
source, transparent region and the observer are in alignment the transparent
region appears uniformly transparent and the image is hidden (Figure 10B).
When the substrate is panned away from the light source the regions of the
transparent region that contain the diffractive optical elements appear
brightly
illuminated but in contrast the non-deflecting region, transmitting ambient
light,
appears dark and the silhouette of the thread is revealed (Figure 10C). When
viewed in reflected light, from either side of the substrate, the silhouette
of the
thread disappears and the holographic image is visible over the whole surface
of
the transparent region (Figure 10D).
Figures 11A-11 D illustrate a security device with a similar two-sided
structure to that described in Figure 10 except that it additionally comprises
a
permanent image formed in a metallised layer 55 applied to the transparent
substrate 31. In this example the metallised design is a fine line pattern.
The first
layer of embossing lacquer 32 is then applied onto the metallised layer 55 and
the optical elements 33,34 subsequently embossed into the lacquer.
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It is known that metallised films can be produced such that no metal is
present in controlled and clearly defined areas. Such partly metallised film
can
be made in a number of ways. One way is to selectively demetallise regions
using a resist and etch technique such as is described in US4652015. Other
techniques are known for achieving similar effects; for example it is possible
to
vacuum deposit aluminium through a mask or aluminium can be selectively
removed from a composite strip of a plastic support and aluminium using an
excimer laser.
On holding the security device in Figure 11 up to a backlight and panning
from side to side the observer will observe the permanent metallised image and
the appearance and disappearance of the silhouette image defined by the non-
deflecting region (Figures 11 B and 11 C). When viewed in reflected light,
from
either side of the substrate, the silhouette disappears and the holographic
image
is revealed over the whole surface of the transparent region in combination
with
the permanent metallised image (Figure 11 D).
The security device in Figure 11 offers three secure aspects; firstly a
permanent image which is not light dependent, secondly a holographic image
viewable only in reflected light and thirdly an optically variable image
viewable
only in transmitted light.
In all of the examples the non-deflecting region and the optical elements
can be inversed such that the resultant silhouette defines the background and
a
negative image is created. Of course, one or more than two optical elements
could be provided.
